Tectonophysics, 49 (1978) 145-148 145

@ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

EARTHQUAKE SOURCE AND PROPAGATION PARAMETERS IN NEW ZEALAND

R.D. ADAMS

New Zealand has a good record of felt reporting of major earthquakes from the latter part of last century, as well as local determinations of Wood- Anderson magnitude (ML) since 1930. This has enabled intensity to be estab- lished as a function of distance and ma~itude over much of the country (Smith, 1976). There are marked regional differences in the form of these relations. An extreme example is in Fiordland, in the southernmost and geo- logically oldest part of the country, where the observed intensities are much lower than might be expected, and should not exceed MM8 even for the largest earthquakes, Historical felt reports are now being used by Mr. G.A. Eiby to reevaluate more accurately the magnitude of pre-instrumental earth- quakes. Two major shocks in central New Zealand in 1848 and 1855 have been assigned magnitudes (~~) of 7.1 and 8.2 respectively.

In general, earthquakes in the east of the North Island do not produce as widespread felt effects as those elsewhere in the North Island, or in the north of the South Island (Fig. I). These regional variations in intensity-distance relations reflect a mixture of source and propagation effects, that must be carefully di.fferentiated in any discussion of earthquake quantification. It has now been established by detailed studies of seismicity that crustal earth- quakes in the eastern part of the North Island are generally deeper than those elsewhere in the country, and that this might result in less generation of surface waves from earthquakes originating in this region.

Gibowicz and Hatherton (1975) have also documented regional differ- ences in the source parameters of shallow earthquakes, that would be expected to affect the spectral content of the generated waves, and thus both the macroseismic effects, and the propagation of earthquake energy. The boundaries between these regions are not closely defined (Fig. 2) and although there are some simil~ities, do not correspond in detail with the source areas defined from intensity studies. What correlations there are, such as in the east coast of the North Island, suggest that earthquakes which pro-

* Now at International Seismological Centre, Newbury and University of Reading, Great Britain.

Fig. 1. Source regions for variations in felt effects (Smith, 1976).

Fig. 2. Variation in source dimension relative to magnitude (Gibowicz and Hatherton, 1975).

duce tighter felt effects have more compact, deeper sources, that would pro- duce a greater proportion of high-frequency energy, and less in the form of surface-waves.

Earthquake measurements will also be affected by propagation conditions. Mr. A.J. Haines has recently studied the attenuation of body waves recorded on short-period seismographs. If the attenuation is expressed as:

Ao(A) = cA-“eYLyA

the propagation over most of the country is consistent with that expected for head waves, with n = 2. In the western and central parts of the North Island, however, rz equals 1 indicating propagation appropriate to body waves (Fig. 3). The attenuation coefficient CY may be neglected in most of the country, corresponding to a value of Q of at least 500. In the central vol- canic region of the North Island, however, the attenuation corresponds to a Q of about 100.

Difficulties with measurements of deep earthquakes are even more pro- nounced. For some deep New Zealand shocks the range of properly defined Wood-Anderson magnitudes is more than 1: units. This is not a station effect, for a different pattern of magnitude residuals may be found for shal- low shocks. These marked propagation effects for deep shocks, apparently

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Fig. 3. Shaded area is where attenuation is consistent with body-wave propagation (n = 1). Elsewhere consistent with head-wave propagation (n = 2). After A.J. Haines.

due to focussing by mantle structure, as well as regional differences in atten- uation, also result in large displacements of the felt patterns. The felt area is centred at the surface projection of the deep earthquake zone, and often does not include the epicentre.

Difficulties in the quantification of earthquakes are compounded in areas of tectonic complexity. Allowance has to be made for local differences in earthquake depth, source characteristics, and propagation effects, including focussing as well as attenuation. Direct determination of seismic moment, by surface-wave methods for shallow shocks, or by spectral analysis, may help to define absolute size, otherwise it may be necessary to directly calibrate magnitudes determined in New Zealand by comparison with teleseismically determined magnitudes of larger shocks.

REFERENCES

Gibowicz, S.J. and Hatherton, T., 1975. Source properties of shallow earthquakes in New Zealand and their tectonic associations. Geophys. J. R. Astron. Sot., 43: 589-605.

Smith, W.D., 1976. Statistical estimates of the likelihood of earthquake shaking through- out New Zealand. Bull. N.Z. Sot. Earthquake Eng., 9: 213-221.

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DISCUSSION OF ADAMS’ PRESENTATION

N. V. Shebalin

Which form of the intensity distance relation was used?

R.D. Adams

We have found different forms of the curve of intensity against logarithm of distance in different parts of the country. In some places it is linear and in others convex upwards. I suspect that this mainly reflects differences in source parameters, including depth.

C. Kisslinger

Have you looked at the duration magnitude for any of these earthquakes, and if so, is there any systematic pattern of variations?

R.D. Adams

No, not for these large shocks, only for microearthquakes. But, this might be important, for the duration magnitude may be related to moment more closely than to the conventional magnitude.

H.S. Hasegawa

In fig. 4 of Kanamori and Anderson (1975) *, seismic moment (MO) is plotted against M, for world-wide earthquakes. For a fixed M,, MO differs depending upon whether the earthquake is of “inter-” or “intra-” plate category. Would the observed variation of MO for a specified M, for New Zea- land earthquakes be due to this phenomenon?

R.D. Adams

The situation in New Zealand is more complicated than this description of inter- or intra-plate classification, as there is not a clear boundary between areas with different magnitude-moment relations.

* Kanamori, H. and Anderson, D.L., 1975. Theoretical basis of some empirical relations in seismology. Bull. Seismol. Sot. Am., 65: 1073-1095.